1. Field of the Invention
This invention relates to an electro-luminescence display (ELD), and more particularly to an electro-luminescence display that is capable of widening an effective display area. Also, the present invention is directed to an ELD that is capable of assuring a sufficient storage capacitance of a capacitor.
2. Description of the Related Art
The ELD is a display device in which electrons and holes are injected from the exterior thereof to re-combine the electrons with the holes and thus produce excited molecules so as to exploit the luminescence of these excited molecules. Since the ELD does not require a backlight, a thin panel can be used and there is a relatively low power consumption. Accordingly, there is a growing interest in a display of this type for use in the future.
FIG. 1 is an equivalent circuit diagram of a unit cell in the conventional ELD. In FIG. 1, a gate line G crosses a data line D to define a pixel cell area. At the pixel cell area, a power supply line L is arranged in parallel to the data line D. The power supply line L may be arranged in parallel to the gate line G. The pixel cell area includes a switching device T1, a driving device T2, a storage capacitor C and an electro-luminescent (EL) diode EL. The switching device T1 has a gate connected to the gate line G, a source connected to the data line and a drain connected to the driving device T2. The drain of the driving device T2 is connected to an anode(+) of the EL diode EL while the source thereof is connected to the power supply line L. The storage capacitor C is connected between the gate of the driving device T2 and the power supply line L. A cathode(xe2x88x92) of the EL diode EL is connected to a common electrode terminal 10.
An operation of the ELD having the structure as described above will be described. If the gate line G connected to the switching device TI is selected by a gate driver (not shown) to be turned on, then a data signal from the data line D connected to the switching device T1 is stored in the storage capacitor C. When the switching device T1 is turned off, a voltage of the storage capacitor C is maintained until the gate line G is selected again. At this time, the storage capacitor C has a voltage applied to the gate of the driving device T2. Thus, a source current determined in accordance with a gate voltage of the driving device T2 arrives at the common electrode 10, via the driving device T2 and the EL diode EL, from the power supply line L. In this operational process, the EL diode EL becomes luminous. In this manner, the driving device T2 responds to a selecting signal applied to the gate line G and the data line D selectively to control a current flowing through the driving T2 from the power supply line L. The EL diode EL controls a magnitude of current with the aid of the driving device T2 and is luminous into a desired magnitude of brightness corresponding to the magnitude of current. For example, if a certain gate voltage is applied to the gate of the driving device T2, then the magnitude of a current passing through the driving device T2 is determined. Accordingly, the magnitude of a current flowing through the diode EL also is determined.
FIG. 2 is a plane view showing the structure of a conventional ELD. Referring to FIG. 2, a gate line 22 crosses a data line 21 to define one pixel cell area. A power supply line 25 is arranged in parallel to the data line 21. A switching device T1 is electrically connected to the data Line 21 and the gate line 22. The switching device T1 consists of an active layer 23, a gate electrode 22G superposed on the active layer 23, a source electrode 21S protruded from the data line 21 and a drain electrode 24 opposed to the source electrode 21S. A driving deviceT2 for driving an EL emitting part 28 is connected to the drain electrode 24 of the switching device T1. A driving device T2 consists of an active layer 27, a gate electrode 26G connected to the drain electrode 24 of the switching device T1, and a source electrode 25S protruded from the power supply line 25. A drain electrode 26D of the driving device T2 is electrically connected to the EL emitting part 28.
A storage capacitor Cap is provided within a pixel cell corresponding to a space between the driving device T2 and the power supply line 25. The storage capacitor Cap uses a portion of a wire connecting the source 25S of the driving device T2 to the power supply line 25 as an upper electrode 25C and uses a wire extended from the gate electrode 26G of the driving device T2 to be superposedwith the upper electrode 25C as a lower electrode 26C.
FIG. 3 and FIG. 4 are section views taken along xe2x80x9cAxe2x80x94Axe2x80x9d and xe2x80x9cBxe2x80x94Bxe2x80x9d lines in FIG. 2, respectively. Referring to FIG. 3 and FIG. 4, the switching device T1 consists of a semiconductor layer 32, a gate insulating film 30, the gate electrode 22G, a film 36 for insulation between layers, the source electrode 21S and the drain electrode 24 which are formed on a substrate 40. The driving device T2 consists of a semiconductor layer 44, a gate insulating film 42, the gate electrode 26G, the film 36 for insulation between layers, the source electrode 25S and the drain electrode 26D which are formed on the substrate 40. The storage capacitor Cap consists of the lower electrode 26C extended from the gate electrode 26G of the driving device T2, and the upper electrode 25C extended from the source electrode 25S of the driving device T2. After the switching device T1, the driving device T2 and the storage capacitor Cap is formed, a protective film 60 is formed in such a manner to cover them. The protective film is provided with contact holes to electrically connect a transparent pixel electrode 70 to the drain electrode 26D of the driving device T2. The pixel electrode 70 is connected to the EL emitting part 28. In other words, the drain electrode 26D of the driving device T2 is electrically connected to the EL emitting part 28. The source electrodes 21S and 25S and the drain electrodes 24 and 26D are coupled with the semiconductor layers 32 and 44 through the contact holes provided within the film 36 for insulation between layers.
The conventional ELD configured as described above uses a wire protruded from the power supply line into the interior of the pixel cell with having a desired area. In such an LCD, an effective display area of the pixel cell is reduced in proportion to an area protruded from the power supply line. Particularly, since an area of the upper electrode must be enlarged so as to increase an accumulated capacitance of the storage capacitor, the effective display area of the pixel cell becomes smaller. Also, a thickness of the film for insulation between layers formed between the lower electrode and the upper electrode of the storage capacitor must be thinned in order to increase a capacitance of the storage capacitor. However, as a thickness of the film for insulation between layers becomes thinner, locations of the source and drain electrodes formed at the same layer as the upper electrode becomes closer to the substrate. In other words, a distance between the source and drain electrodes and the gate electrode is decreased in order to each other to increase a parasitic capacitance. To the contrary, if a thickness of the film for insulation between layers becomes larger to reduce a parasitic capacitance, then an accumulated capacitance of the storage capacitor is decreased so that it is impossible to accumulate the level of voltage required for a driving of the pixel cell.
Accordingly, it is an object of the present invention to provide an electro-luminescence display that is capable of widening an effective display area of a pixel.
A further object of the present invention is to provide an electro-luminescence display that is capable of assuring a sufficient storage capacitance of a capacitor.
In order to achieve these and other objects of the invention, an electro-luminescence display according to one aspect of the present invention includes a gate line; a data line formed in a direction crossing the gate line; a power supply line formed in a manner such that it is insulated from the gate line and the data line; a first switching device having a gate connected to the gate line and a source connected to the data line; a second switching device having a gate connected to a drain of the first switching device and a source connected to the power supply line; and electric emitting part connected to the drain of then second switching device; and a storage capacitor formed in a longitudinal direction of the power supply line to charge a voltage applied to the gate of the second switching device.
An electro-luminescence display according to another aspect of the present invention includes a gate line; a data line formed in a direction crossing the gate line; a power supply line formed in such a manner to be insulated from the gate line and the data line; a first switching device having a gate connected to the gate line and a source connected to the data line; a second switching device having a gate connected to a drain of the first switching device and a source connected to the power supply line; and a storage capacitor including an upper electrode and a lower electrode superposed with having a dielectric layer therebetween to charge a voltage applied to the gate of the second switching device, said upper electrode of the storage capacitor being formed on a dielectric layer different from the source and drain electrodes of the second switching device.